The world is full of sensor devices for detecting physical phenomenon and for providing a signal in response to the phenomenon. For example, a thermometer converts the physical condition temperature into a visual signal, a height of mercury in a glass column. Another example of a temperature-sensing device is a thermocouple which converts the physical condition temperature into an electrical signal. To be useful the sensor signal has to be understood to correspond with a particular physical phenomenon. For example, the thermometer has lines on the glass column to indicate the degrees of temperature. The lines, of course, have to be in the right locations on the glass column to have meaning, and the process by which the lines are properly located is known as calibration. During calibration the sensor is subjected to a known physical condition or conditions and its response is observed. Observing the response of the sensor to the known conditions allows one to predict the sensor response for a wide range of conditions.
Pressure sensors are devices that provide a signal indicative of pressure, for example, the amount of air pressure within a tire. As with other types of sensors, pressure sensors require calibration to be useful. A specific kind of pressure sensor known as a piezoresistive pressure sensor provides a voltage signal indicative of pressure. The piezoresistive pressure sensor poses a number of problems in application. For example, the piezoresistive sensing element provides a relatively low level voltage signal. In addition, the piezoresistive sensing element may provide a signal that is sensitive to changing temperature and that does not change linearly with changing pressure. Moreover, the signal voltage characteristic from one sensing element to another sensing element may not be consistent. Therefore, special signal conditioning circuitry is required for a sensor product that provides a high level sensor output that is sufficiently accurate across a wide range of operating temperatures and pressures. Importantly, the device has to be capable of mass production, at low cost and with a high degree of part-to-part repeatability.
Most low cost signal conditioning approaches use analog circuits that are adjusted during a calibration process. For example, it is known to use amplifier circuits coupled to resistor networks. In one such application, the resistor network includes a number of resistive elements coupled by fusible links. Though limited in the degree of adjustment available, various resistive values may be established for providing an acceptable output from the amplifier network. In another application, the resistor network includes laser trimmable resistive elements. During a calibration process, the resistive elements are trimmed using a laser to achieve the correct resistive values to provide an acceptable output from the amplifier network. In either application access to the circuit may be required during processing in order to fuse links and/or laser trim components. Hence manufacturing processing options are limited. Also, in certain applications offset, sensitivity and linearity may be difficult to compensate for independently. Furthermore, processing activities following calibration may introduce error that can not be corrected in the final product. And, the laser trim process requires expensive processing hardware and suffers increased cycle time.
An alternative design provides for electronic calibration of the sensing element. Sensors adapted for electronic calibration have included a microprocessor coupled to the sensor element via suitable signal conditioning circuitry and to a memory in which a calibration algorithm is retained. During processing, the sensing element is tested under various known-operating conditions. Calibration values are established and stored in the memory. In operation, these data are accessed and utilized by the sensor to provide a reliable sensor output. The memory is typically an electronically erasable programmable read only memory (EEPROM) to which the calibration data may be written during manufacture of the sensor or once installed in an application. As a result of the flexibility in application and ease of manufacture, electronically calibrated sensors are highly desirable.
It is common for electronically calibrated sensors to find application in harsh environments. For example, pressure sensors find application in industrial processing plants, internal combustion engine controllers, and the like. It is not unusual for these environments to not only be physically harsh but electronically noisy. For example, in applications for controlling internal combustion engines in automobiles, the electrical environment may include ignition spark noise, fuel-injector driver noise and high power pulse-width modulation motor controller noise to name just a few forms of electrical noise to which the sensor may be subjected. It is therefore very important that the calibration data stored in the memory be protected from corruption and/or accidental erasure resulting from exposure to the electronic noise.
Inadvertent writes or erases of an EEPROM device may occur from a random power-up state of the write/erase digital control logic which forms part of the device. Data may also be accidentally overwritten if the proper sequence of external random events on the accessible pins of the sensor module places it in a write or erase mode. Such random events may result from electrical noise in the sensor operating environment.
A number of strategies have been employed to protect the memory of electronic devices that utilize EEPROM memory. For example, to protect against inadvertent writes or erases resulting from random power-up states of the write/erase digital control logic, a power-on reset (POR) circuit is often used. The POR ensures that the write/erase digital control logic always powers on in a safe, write protected mode. However, it is very difficult to design a POR to protect against all possible electronic noise conditions, the POR will require valuable die area in the circuit design, and the POR only protects against power-on conditions and not against conditions that may occur after power-up.
Certain prior art teach an electronically erasable (EE) fuse structure within a memory device to maintain the secrecy of the data. The data is inaccessible with the fuse set. Resetting the fuse results in erasure of the data. This arrangement does not provide protection of the data from unauthorized or inadvertent writing or erasure of the data.
Therefore, there remains a need for a cost effective electronically calibrated sensing device including a protected EEPROM . The preferred device will provide robust protection for data written to the EEPROM under all operating conditions, and including highly electronically noisy conditions. In addition, the device will be efficient in its circuit implementation, accept allowed reprogramming, be directly testable and not require additional pins when packaged in an integrated circuit (IC) package.